Perhaps it might be better to word this, "Determine for whom the message is intended." This may seem less important than the speaker, but in many cases it is equally important.

Under What Law Spoken?

This is the same as was found in our consideration of determining the speaker. I recommend that you read that section if you have not already.

Christian or non-Christian?

I have many times found people appealing to statements made to erring Christians in the Bible, as if they applied to non-Christians. For example, Simon believed and was baptized in Acts 8:13. Later, when he has sinned, Peter tells him to repent and pray for forgiveness (8:22). Some wish to cite that passage as a case of an alien sinner becoming a Christian by prayer to God. They have not determined the state of the recipient. Simon had already been born again, and was already a child of God. He did not need to be born again, again. He needed to repent and pray for forgiveness from his Father.

The case of conversion in Acts 2:38 illustrates the need to look at the intended listener, Peter tells the multitude to repent and be baptized. Did they not have to believe? It is clear from an examination of the text that they already believed (the were pricked in their heart - v37). The epistles of the New Testament were written to saved people. But many time unsaved people will read them and think that the descriptions therein apply to them. Ephesians 2:8f are a good example of this. Some people read, "For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast." and think that it is referring to them. But the book of Ephesians was written to saints. It was written to people who have complied with God's will concerning salvation. Jesus told the rich young ruler to sell all and follow Him (Lk 18:22). It is not until we recognize that the hearer of that command had a particular problem that we begin to see that it is not intended for all the world to obey.

Conclusion...

Be certain that the circumstance of the listener in the Biblical record matches yours before you try to apply his commands to yourself. The one for whom a message is intended is an important aspect of understanding the Bible.